ARMA/USRMS 05-671
American Rock Mechanics Association, 2005. - 9 p.This paper was prepared for presentation at Alaska Rocks 2005, The 40th U.S. Symposium on Rock Mechanics (USRMS): Rock Mechanics for Energy, Mineral and Infrastructure Development in the Northern Regions, held in Anchorage, Alaska, June 25-29, 2005.ABSTRACT: Molas Pass is located in Southwestern Colorado on US
550. In August 2003, Colorado Department of Transportation (CDOT) began a rockfall mitigation project to all eviate potential dangers caused by rockfall and/or rockslides and to improve the sight distance and reduce the blind corner caused from the rockslope. The work involved rock excavation by production blasting, rock reinforcement with dowels and rock bolts and rockfall mitigation using wire mesh drape. A contractor’s access road was constructed across the upper rock face about 85 feet above the shoulder of the highway to facilitate excavation of the rockslope. The excavation created an unstable highwall specifically a large rock block above the access road. Blasting of the access road exacerbated the rockslope instability of the high wall by day lighting the failure plane and unweighting the toe of the slope of the highwall. The stability of the highwall was aggravated by the excessive backbreak created from blasting. A combination of factors in the blasting such as over-stemming of the shot holes and short timing delays in the firing sequence between the second and back shot hole rows may have led to the severe backbreak. Moreover, the adverse geologic structure appears to have exacerbated the excessive backbreak.
